1. Field of the Invention
The present invention relates to a solar cell module which excels in weatherability, heat resistance, moisture resistance, scratch resistance, and has an excellent protective ability against external pressure. More particularly, the present invention relates to a solar cell module comprising a photovoltaic element (that is, a solar cell) enclosed by a filler material and a coating material having a three-layered structure which is disposed on the light receiving surface of the solar cell, wherein said coating material is composed of a layer made of hard resin having a hardness of 50 or more in Shore hardness D (hereinafter, referred to as a hard resin layer); a layer having a function of absorbing ultraviolet rays liable to deteriorate the hard resin layer and capable of transmitting light necessary for photoelectric conversion of the solar cell and also having an adhesive function (hereinafter, referred to as an adhesive layer); and a layer made of a resin excellent in weatherability (the resin itself being stable against heat, light, and moisture; hereinafter, referred to as an outermost layer), said hard resin layer, adhesive layer, and outermost layer being laminated on the light receiving surface side of the photovoltaic element in this order from the light receiving surface side. The coating material having the above-described three-layered structure in the solar cell module of the present invention prevents damage to the photovoltaic element when applied with external pressure, and provides the weatherability, heat resistance, moisture resistance, and scratch resistance necessary for the solar cell module.
2. Related Background Art
Recently, a number of thin film solar cells have been proposed. A typical one of these thin film solar cells is an amorphous silicon (a-Si) thin film solar cell. As for the a-Si thin film solar cell, there is known a type in which an a-Si semiconductor film functioning as a photoelectric conversion element is provided on a conductive substrate and a transparent conductive layer is provided on the semiconductor thin film. In the case where the a-Si thin film solar cell having the above construction is used as a power supply means, the surface of the a-Si solar cell on the light incident side must be protected, unlike a solar cell having a construction using a glass sheet as the substrate. For this purpose, a protective means is provided on the surface of the a-Si solar cell on the light incident side. It is most important for the protective means to sufficiently transmit sunlight in order to maintain the conversion efficiency of the solar cell. The protective means is also required to protect the interior of the solar cell against wind and rain and the other external influences (hereinafter, referred to as interior protective ability). It is also important that the protective means itself be prevented from being deteriorated, discolored, and reduced in mechanical strength due to light, heat, and moisture. As such a protective means, there is known a type in which a transparent resin layer of excellent weatherability is provided on the light receiving surface side as a surface coating layer, and a filler material made of thermoplastic transparent resin is provided below the transparent resin layer. The resin layer as the surface coating layer is generally composed of an acrylic resin film or a fluororesin film such as a tetrafluoroethylene-ethylene copolymer film or polyvinyl fluoride film. As described above, a filler material is interposed between the surface coating layer and the solar cell. As the filler material, there is generally used EVA (ethylene-vinyl acetate copolymer), butyral resin, or the like. A back surface film is provided on the back surface of the conductive substrate of the solar cell by means of a filler material. As the back surface film, there is used a nylon film, a fluororesin laminated aluminum film, or the like. Moreover, in a practical solar cell module, a reinforcing material is provided on the back surface of the back surface film by means of a filler material. Hence, each of the filler materials interposed between the surface coating layer and the solar cell and between the conductive substrate and the back surface film must function as an adhesive and while protecting the photovoltaic element from scratch damage and exterior impacts.
The interior protective ability of the protective means having the above construction, however, is dependent on the thickness of the coating material composed of the surface coating layer on the light receiving surface and the filler material. Specifically, as the thickness of the coating material is increased, the interior protective ability is increased; and as it is decreased, the interior protective ability is reduced. However, when the thickness of the surface coating layer on the light receiving surface side is increased, separation tends to occur at the interface between the surface coating layer and the filler material due to temperature change. The separation causes a problem in that moisture reaches the solar cell through the separated portion, and thereby not only the characteristics of the solar cell are reduced but also a leakage current is generated due to permeation of moisture. The filler material which encloses the solar cell must fill the irregularities of the solar cell and adhere to the surface coating layer. The filler material is thus required to have a rubber elasticity.
Moreover, as the thickness of the surface coating material is increased, the light transmissivity thereof is reduced, which lowers the conversion efficiency of the solar cell. The solar cell module having the above construction is generally manufactured in the following procedure. Namely, a resin film as the front surface coating layer on the light receiving surface side, a front surface filler material, a solar cell, a back surface filler material, and a back surface film are laminated, and then hot-pressed using a vacuum laminator. In this manufacturing method, since the end portions of the solar cell module come into close-contact upon hot-pressing, air sometimes remains in part of the interior between the filler material and the surface coating material on the light receiving surface side and between the filler material and the solar cell. As a result, bubbles often remain in the sealed solar cell module. The bubbles thus remaining in the solar cell module are repeatedly expanded and contracted due to temperature changes, thus leading to the separation of the coating material. The thus generated separation causes the above-described problem that moisture reaches the solar cell through the separated portion, whereby the conversion efficiency is reduced. Moreover, the remaining bubbles deteriorate the appearance of the solar cell module, thereby reducing the yield of products.
As a means for solving the above-described problem, there is known a method of inserting unwoven glass fiber fabric between the filler material and the surface coating layer on the light receiving surface side and between the filler material and the solar cell, and then laminating them. In this method, the surface coating material is reinforced by glass fibers, and it is thus improved in its mechanical strength. The interior protective ability of the surface coating material against an external force is increased because of the increased mechanical strength, and thereby the thickness of the surface coating material can be reduced. This makes it possible to prevent the above separation which more readily occurs when the thickness of the surface coating layer on the light receiving surface side is large. Moreover, since the thickness of the surface coating material can be relatively thin, the reduction in the light transmissivity of the surface coating material the reduction in the conversion efficiency of the solar cell are suppressed.
In the manufacturing process of the solar cell module, since glass fibers are interposed between the filler material and the solar cell and between the solar cell and the surface coating layer on the light receiving surface side, even when the solar cell module is pressed, air vent passages can be ensured at the end portion of the solar cell module, thereby facilitating evacuation and elimination of the remaining bubbles.
In this case, however, the glass fibers are inevitably exposed at the end portion of the solar cell module, and thereby moisture easily permeates into the solar cell module by way of the glass fibers, the permeating moisture sometimes exerting adverse effects on the solar cell. To prevent the glass fibers from being exposed at the end portion of the solar cell module, there may be considered a method of previously cutting the glass fibers into sizes smaller than that of the solar cell module; however, in this case, the end portion of the solar cell module would lack air vent passages similarly to the structure with no glass fibers, thereby generating the bubbles. This method, therefore, fails to sufficiently solve the above-described problem.